Hydraulic fracturing is a term that has been applied to a variety of methods used to stimulate the production of fluids such as oil, natural gas, brines, etc., from subterranean formations. In hydraulic fracturing, a fracturing fluid is injected through a wellbore and against the face of the formation at a pressure and flow rate at least sufficient to overcome the overburden pressure and to initiate and/or extend a fracture(s) into the formation. The fracturing fluid usually carries a proppant such as 20-40 mesh sand, bauxite, glass beads, etc., suspended in the fracturing fluid and transported into a fracture. The proppant then keeps the formation from closing back down upon itself when the pressure is released. The proppant-filled fractures provide permeable channels through which the formation fluids can flow to the wellbore and thereafter be withdrawn.
Hydraulic fracturing has been used for many years as a stimulation technique and extensive work has been done to solve problems present at each stage of the process. For example, a fracturing fluid is often exposed to high temperatures and/or high pump rates and shear which can cause the fluids to degrade and to prematurely drop the proppant before the fracturing operation is completed. Considerable effort has, therefore, been spent trying to design fluids that will satisfactorily meet these rigorous conditions.
High permeability formations such as those having permeabilities in excess of 50 millidarcy and particularly in excess of 200 millidarcy, present special challenges, especially when the reservoir temperature is above 1300.degree. F. In these situations, the amount of fluid lost to the formation can be very high, resulting in increased damage and decreased fracture length. Further, the difference in permeability between the formation and the fracture is less than that realized in less permeable formations. Improved fracture cleanup is therefore necessary in order to maximize well productivity.
A wide variety of fluids has been developed, but most of the fracturing fluids used today are aqueous based liquids which have been either gelled or foamed. These fluids have typically been engineered for use in low permeability formations and are generally not well suited for use in higher permeability formations.
Aqueous gels are usually prepared by blending a polymeric gelling agent with an aqueous medium. Most frequently, the polymeric gelling agent of choice is a solvatable polysaccharide. These solvatable polysaccharides form a known class of compounds which include a variety of natural gums as well as certain cellulosic derivatives which have been rendered hydratable by virtue of hydrophilic substituents chemically attached to the cellulose backbone. The solvatable polysaccharides therefore include galactomannan gums, glycomannan gums, cellulose derivatives, and the like. In some instances, the aqueous liquid thickened with polymers alone has sufficient viscosity to suspend the proppant during the course of the fracturing process and represents a satisfactory fracturing fluid. In other instances, principally in higher temperature applications, however, it is necessary to crosslink the polysaccharide in order to form a gel having sufficient strength and viscosity to retain the proppant in suspension throughout the pumping operation and placement in the subterranean formation. A variety of crosslinkers (e.g., borate ions, titanium, zirconium and other transition metals) has been developed to achieve this result within different pH ranges. (See, for example, U.S. Pat. No. 3,059,909, U.S. Pat No. 3,974,077, U.S. Pat No. 3,202,556, U.S. Pat. No. 3,301,723 and U.S. Pat. No. 3,888,312.)
A different class of thickeners is described in U.S. Pat. No. 4,432,881, and identified as a superior fracturing fluid in U.S. Pat. No. 4,541,935. The thickener composition comprises a water soluble or water dispersible interpolymer having pendant hydrophobic groups chemically bonded thereto. When mixed with a water soluble or water dispersible nonionic surfactant, and a soluble electrolyte, a viscosified fluid stable to high temperature and/or shear is obtained.
Viscoelastic surfactants are employed as viscosifiers in the context of gravel packing fluids. Such systems contain virtually no insoluble residue. Gravsholt in Viscoelasticity in Highly Dilute Aqueous Solutions of Pure Cationic Detergents, J. Colloid & Interface Sci. (57)3(1976), 575-77 indicates that certain quaternary ammonium salts impart viscoelastic properties to aqueous solutions. Gravsholt showed that cetyl trimethyl ammonium bromide would not impart viscoelastic properties to water but that cetyl trimethyl ammonium salicylate and certain other aromatic containing quaternary amines would. In U.S. Pat. No. 3,292,698, a mixture of cyclohexyl ammonium chloride and undecane-3-sodium sulfate was taught to induce viscoelastic properties to a formation flooding liquid containing less than about 3.5 percent by weight of sodium chloride. Higher levels of sodium chloride were said to destroy the viscoelastic properties of the fluid. UK Pat. No. 1,443,244, discloses a specific ethoxylated or propoxylated tertiary amine employed to thicken and aqueous solution of a strong mineral acid. U.S. Pat. No. 3,917,536 teaches that certain primary amines may be employed in subterranean formation acidizing solutions to retard the reaction of the acid on the formation. The amine may be more readily dispersed into the acid solution with the use of a dispersing agent such as a quaternary amine.
In particular, Canadian Pat. No. 1,185,779, discloses a high electrolyte-containing aqueous wellbore service fluid which has improved viscosity characteristics over a wide range of wellbore conditions, including improved ease of preparation at the wellside and better shear stability and consistent viscosity over a wide temperature range. These improved aqueous wellbore service fluids are acknowledged as being useful in well known wellbore services such as perforation, clean-up, long term shut-in, drilling, placement of gravel packs and the like.
U.S. Pat. No. 5,551,516 discloses a fracturing fluid that contains (a) an aqueous base fluid, (b) an inorganic water soluble salt, (c) a thickener selected from certain amines, amine salts or quaternary ammonium salts and (d) a stabilizing organic additive. However, there is no disclosure therein of how to make the thickening agents, and therefore, no disclosure relating to the importance of controlling reaction conditions or process parameters (such as, for example, the presence or absence of intermediates in the final product) employed during manufacture of the thickening agent.